Structure features and phase behaviour of amphiphilic $N$-tetradecyl-$\beta$-hydroxy-propionic acid amide monolayers

Two phase transitions of N-tetradecyl-β-hydroxy-propionic acid amide monolayers at the air/water interface were studied by means of pressure/surface area (πA) isotherms, Brewster angle microscopy (BAM) and synchrotron X-ray grazing-incidence diffraction (GID). At T≤15 °C the πA isotherms show two plateau regions indicating two first-order phase transitions. In the first main transition from the lowdensity fluid-like phase to a condensed phase, dendritic growth of condensed phase domains with six main growth directions was observed by BAM. A second transition from a condensed phase with large tilt angle to a condensed phase with smaller tilt angle and a denser arrangement of molecules was found. The molecular area decreases jump-like by a value of ca. 0.02 nm2. The diffraction patterns (GID) of the low-density condensed phase show three peaks. In this state the crystal lattice of the monolayer is oblique. The azimuthal tilt direction of alkyl chains is close to nearest neighbours. The amphiphilic molecules change their conformation abruptly in a second transition. In the region above the second phase transition at high surface pressures, the crystal lattice remains oblique, but the molecules occupy a smaller molecular area parallel to the water surface and the tilt direction of the molecules is changed abruptly to an intermediate tilt direction. The balance of interactions between the polar head groups, dominated by hydrogen bonding between the amide and hydroxyl groups of the acid amide moieties, influences the morphological features of domains and the crystal structure of these monolayers, as well as the conformation of molecules at the air/water interface